This invention relates to methods and apparatus for recording and/or reproducing compressed data, that is bit-compressed digital audio signals or the like, methods and for transmitting and/or receiving the compressed data, and a recording medium therefor. More particularly, it relates to an apparatus, technique and a recording medium for recording with the compression mode of plural bit rates.
The present Assignee has already proposed in our U.S. Pat. Nos. 5,243,588 and 5,244,705 a technique consisting of bit-compressing input digital audio signals and recording the bit-compressed signals in a burst fashion with a pre-set data volume as a recording unit.
This technique resides in employing a magneto-optical disc as a recording medium and recording and/or reproducing AD (adaptive differential) PCM audio data as prescribed in audio data formats such as CD-I (CD-Interactive) or CD-ROM XA. The AD-PCM audio data is recorded in a burst fashion on the magneto-optical disc with, for example, 32 sectors of the AD-PCM data and a few linking sectors for interleaving, forming one recording unit.
Several modes may be selected of the AD-PCM audio data in the recording and/or reproducing apparatus employing the magneto-optical disc. For example, a level A with a compression ratio twice that of the conventional compact disc (CD) and a sampling frequency of 37.8 kHz, a level B with a compression ratio four times that of the conventional compact disc (CD) and a sampling frequency of 37.8 kHz and a level C with a compression ratio eight times that of the conventional compact disc (CD) and a sampling frequency of 18.9 kHz, are prescribed. That is, with the above-mentioned level B, the digital audio data is compressed to approximately one/fourth of the original volume, with the replay time of the disc recorded with the level B mode being four times that with the standard CD format. This indicates that the apparatus may be reduced in size because a recording/reproducing time about as long as that achieved with a standard size disc of 12 cm in diameter may be achieved with a smaller sized disc.
However, since the rotational speed of the disc is the same as that of a standard CD, the amount of the compressed data obtained with the level B per a pre-set time is four times that of the standard CD. Consequently, the same compressed data is repeatedly read four times, in a time unit of, for example, a sector or a cluster, and only one of the four superimposed compressed data is transmitted to audio reproduction. Specifically, during scanning or tracking of a spiral recording track, a track jump to return to the starting track position is performed for each revolution in order to repeatedly track the same spiral track four times. This indicates that at least one of the four scans of compressed sound data from the repeated reading operations suffices. Hence, the disclosed technique is effective against errors caused by disturbances, and consequently may be especially suitable for use in connection with a small-sized portable type device.
Since, a semiconductor memory will be employed in the future as a recording medium, it would be desirable to achieve further bit compression in order to improve compression efficiency. Specifically, audio signals are recorded and/or reproduced using a so-called IC card (which includes one or more such semiconductor memories), on or from which bit-compressed data is recorded or reproduced, respectively.
With the IC card, increases in storage capacity or lower costs are achieved through progress in semiconductor technology. Notwithstanding such progress, when the IC card was first supplied to the market, the IC card was believed to have inadequate storage capacity, and furthermore, was expensive. Therefore, it may be contemplated to repeatedly transfer the recording contents of an inexpensive large-capacity recording medium, such as the magneto-optical disc, to the IC card by re-writing data a number of times. Specifically, desired musical numbers recorded on the magneto-optical disc are dubbed to the IC card and exchanged (or overwritten) with other musical numbers already stored in the IC card. By frequently re-writing the contents of the IC card in this manner, a wide variety of musical numbers may be enjoyed outdoors with a small number of IC cards which are on hand.
For recording/reproducing audio signals, however, the frequency bandwidths and the signal to noise characteristics differ for a variety of applications. For example, if high quality audio signals are desired, the bandwidth of from 15 kHz to 20 kHz and a good signal to noise ratio are required. For such purposes, a higher bit rate is tolerated. The bit rate in this case is usually 256 kbps to 64 kbps per channel.
Conversely, if, for the most pan, audio signals are handled, a bandwidth of 5 to 7 kHz suffices, while a significantly high signal to noise ratio is not required. However, in order to increase recording/reproducing time for a given memory size, it is desirable to lower the bit rate from 64 kbps to several kbps. In light of the above, it is necessary to provide a recording/reproducing apparatus which is capable of accommodating the plural usages having different demand levels and in which it is possible to reduce the economic load to the maximum extent possible. However, if plural modes having different bandwidths have to be provided, it therefore is necessary to support plural sampling frequencies, with the result that the sampling frequency signal generating circuit tends to be complex or the LSI scale tends to be unavoidably increased. In addition, if the sampling frequency differs from one mode to another, it is difficult to transfer the information from one mode to another. If it is desired to write high bit rate mode information recorded on a large-capacity magneto-optical disc onto a small-capacity IC card at a low bit rate mode, it is necessary to completely annul the compression mode to restore the information to time-domain signals and to re-compress the signals at a low bit rate mode, with the consequence that the quantity of the arithmetic-logical steps is increased, thereby increasing the difficulty of real-time processing.
With the modes having increasingly low bit rates, however, the sound quality is lowered due to the decrease in the number of usable bits. If, when the bandwidth is narrowed, the bandwidth of the frequency division for compression is constant over the entire frequency range, the bandwidth resulting from division of the entire frequency range of 20 kHz into 32 bands of equal widths is 700 Hz, which is significantly wider than the low-range critical band of 100 Hz and wider than the critical bands for the major portion of the frequency range. The result is the significantly lowered compression efficiency. However, if, when the bit rate is lowered, bit reduction is made preferentially for one of the main information and the subsidiary information, the sound quality is lowered significantly. For this reason, it becomes necessary to reduce not only the main information but also the subsidiary information as well.